1. Field of the Invention
The present invention relates to a micropower RC oscillator which can secure a stable current source, use a variable resistor and a variable capacitor such that the system-on-chip of passive elements can be realized, and can stably operate with respect to changes in temperature and process.
2. Description of the Related Art
In general, oscillator circuits serve to provide a clock signal or timing signal to electronic circuits such as a microprocessor, a micro controller, a flip-flop circuit, a latch circuit and the like and are widely applied to various fields in the electronic industry. An accurate and stabilized reference frequency can be obtained using a crystal oscillator circuit.
In many application fields, however, such a high-quality reference frequency is not need, and mass productivity needs to be considered. Therefore, an RC oscillator circuit can serve as a low-cost clock signal source or timing signal source. Further, such an RC oscillator circuit can generate a variable frequency by varying resistance R or capacitance C, and inductors which are hard to process into an integrated circuit can be prevented from being used in the RC oscillator circuit.
In mobile systems such as mobile terminals or portable electronic apparatuses using batteries, ultra-low current consumption is required. Therefore, micropower RC oscillators are advantageous in such an application field, because low power consumption is advantageous in enlarging the lifetime of a battery.
As other requirements for RC oscillators, operation characteristics with respect to changes in temperature and process, or particularly, an oscillation frequency characteristic should be stable. Further, the RC oscillators should be able to be implemented simply and at a low cost through a configuration in which passive elements with a relatively large size are disposed in a chip.
FIG. 1 is a circuit diagram of a conventional RC oscillator. As shown in FIG. 1, the conventional RC oscillator 100 roughly includes a current generating section 110, a charge and discharge circuit section 120, and an oscillation signal output section 130.
The current generating section 110 generates a current and supplies the generated current through a mirroring section 140.
The charge and discharge section 120 includes a capacitor C and a plurality of transistors 121 to 123. The charge and discharge section 120 charges and discharges the capacitor C by using the current generated from the current generating section 110 and the plurality of transistors 121 to 123.
The oscillation signal output section 130 connected to the charge and discharge circuit section 120 includes a plurality of inverters 131 and 132 and outputs an oscillation signal RC_OSC_OUT of which the frequency is determined by a resistor R and the capacitor C.
The RC oscillator configured in such a manner operates as follows.
When a power supply voltage VDD is applied, the current generating section 110 is operated to generate a current, and the generated current is supplied to a current source through the mirroring section 140.
Then, a voltage is charged into the capacitor C by the supplied current. The charged voltage drives a first inverter 131 of the oscillation signal output section 130 through the transistor 121 of the charge and discharge circuit section 120.
Further, a signal output from the last inverter 132 of the oscillation signal output section 130 is fed back to operate the transistors 122 and 123 of the charge and discharge circuit section 120. Accordingly, as the capacitor C is discharged, an oscillation signal RC_OSC_OUT having a rectangular clock shape is output.
In the conventional RC oscillator, however, the power supply voltage is directly applied as a driving power supply of the plurality of inverters outputting an oscillation signal such that current consumption increases. Accordingly, the lifetime of battery of a mobile system using the RC oscillator is reduced.
Further, resistance and capacitance cannot be changed freely so that the system-on-chip of passive elements is impossible. Accordingly, a large cost is required for implementing such an RC oscillator. The RC oscillator sensitively reacts with changes in temperature and process, thereby outputting an unstable oscillation signal.
In addition, a bias operating point can be fixed to zero (0) such that the RC oscillator itself may be not operated.
FIG. 2 is a graph showing a current ID of the current generating section in accordance with a bias voltage VGS. As shown in FIG. 2, bias operating points on a load line are represented as two points A and B. If the point A with a value of zero is fixed to the bias operating point, the RC oscillator itself may not be operated.